My first airline was non BALPA, but a few pilots were members. While the airline was in administration, but before it went into receivership, the pilots were all made redundant with no notice or pay in lieu. BALPA's advice? Roll over and let them tickle your tummy. A couple of pilots used the legal advice available with their house insurance to seek a second opinion. As a direct result of their efforts the law was retrospectively changed 5 years later, and the pilots were paid in lieu of notice. Surely this would have been a perfect test case for BALPA to have taken up in the first place. Unfortunately we were just a small regional airline and they didn't bother.
Notwithstanding the above I later joined BALPA for the security of membership. As a First Officer the fee (in £ not %) seemed a reasonable sum of money. However, later on as a Captain and then training Captain, the fee (again in £ not %) seemed exhorbitant. So I wrote to BALPA to resign.
The reply from BALPA was staggering in its arrogance. My application to resign would be considered at the next commitee meeting! Hello, I've already gone and my direct debit is cancelled, you can discuss it untill you're blue in the face, but I ain't coming back.
I would like to think that BALPA has learn't a few lessons since then and is more inclusive, but I'm quite comfortable on the outside and have no plans to return.
I have not tried the IPA/IPF, but would do so before I went back to BALPA.